Tyrosine kinases play a critical role in signal transduction for several cellular functions including cell proliferation, carcinogenesis, apoptosis, and cell differentiation. Inhibitors of these enzymes are useful for the treatment or prevention of proliferative diseases which are dependent on these enzymes. Strong epidemiologic evidence suggests that the overexpression or activation of receptor protein tyrosine kinases leading to constitutive mitogenic signaling is an important factor in a growing number of human malignancies. Tyrosine kinases that have been implicated in these processes include Abl, CDK's, EGF, EMT, FGF, FAK, Flk-1/KDR, Flt-3, GSK-3, GSKbeta-3, HER-2, IGF-1R, IR, Jak2, LCK, MET, PDGF, Src, Tie-2, TrkA, TrkB and VEGF. Hence, there is an ongoing need to investigate novel compounds that can be used to regulate or inhibit tyrosine kinase enzymes. Inhibitors of tyrosine kinase enzymes may be used to treat diseases which are characterized by an overexpression or upregulation of tyrosine kinase activity such as cancer, diabetes, restenosis, arteriosclerosis, psoriasis, angiogenic diseases and immunologic disorders (Powis, G.; Workman P. Signaling Targets For The Development of Cancer Drugs. Anti-Cancer Drug Design (1994), 9: 263-277; Merenmies, J.; Parada, L. F.; Henkemeyer, M. Receptor Tyrosine Kinase Signaling in Vascular Development. Cell Growth Differ (1997) 8: 3-10; Shawver, L. K.; Lipsosn, K. E.; Fong, T. A. T.; McMahon, G.; Plowman, G. D.; Strawn, L. M. Receptor Tyrosine Kinases As Targets For Inhibition of Angiogenesis. Drug Discovery Today (1997) 2: 50-63; all herein incorporated by reference).
Receptor tyrosine kinases (RTKs) are important in the transmission of biochemical signals across the plasma membrane of cells. These transmembrane molecules characteristically consist of an extracellular ligand-binding domain connected through a segment in the plasma membrane to an intracellular tyrosine kinase domain. In general, RTKs are activated by ligand-induced oligomerization and tyrosine autophosphorylation of specific intracellular substrates such as PLCγ, PI3 kinase, ras, and raf MEK/Erk1. Tyrosine kinase activity is an absolute requirement for signal transduction through this class of receptor.
Tropomysosin Related Kinases (Trk) are a family of receptor tyrosine kinases composed of three family members, TrkA, TrkB and TrkC. The Trks bind with high affinity to, and mediate the signal transduction induced by the Neurotrophin family of ligands whose prototype members are Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF) and Neurotrophin-3, -4 and -5 (NT-3, NT-4 and NT-5). In addition, a co-receptor lacking enzymatic activity, p75, has been identified which binds all neurotrophines (NTs) with low affinity and regulates neurotrophin signaling. A critical role of the Trks and their ligands during the development of the central and peripheral nervous systems have been established through gene disruption studies in mice. In particular, TrkA-NGF interaction was shown as a requirement for the survival of certain peripheral neuron populations involved in mediating pain signaling. In addition to these developmental consequences of Trk signaling, the subversion of this receptor and its signaling pathway in certain malignancies has also been documented. Of particular note are reports of aberrant expression of NGF and TrkA receptor kinase are implicated in the development and progression of human prostatic carcinoma and pancreatic ductal adrenocarcinoma and activating chromosomal rearrangements of Trks in acute myelogenous leukemia (AML), thyroid and breast cancers and receptor point mutations predicted to be constitutively activating in colon tumors. In addition to these activation mechanisms, elevated Trk receptor and ligand have also been reported in a variety of tumor types including multiple myeloma, melanoma, neuroblastoma, ovarian and pancreatic carcinoma. The neurotrophins and their corresponding Trk receptor subtypes have been shown to exert a variety of pleiotropic responses on malignant cells, including enhanced tumor invasiveness and chemotaxis, activation of apoptosis, stimulation of clonal growth, and altered cell morphology. These effects have been observed in carcinomas of the prostate, breast, thyroid, colon, malignant melanomas, lung carcinomas, glioblastomas, pancreatic carcinoids and a wide variety of pediatric and neuroectodermal-derived tumors including Wilm's tumor, neuroblastomas and medulloblastomas. Neurotrophins and their receptor subtypes have been implicated in these cancers either through autocrine or paracrine mechanisms involving carcinoma cells and the surrounding parenchymal and stromal tissues. In addition, profound or significantly attenuated reduction of bone pain caused by prostate cancer metastasis has recently been achieved by utilization of anti-NGF antibody. Overall, the oncogenic properties of Trk signaling in multiple tumor types makes the modulation of the Trk receptor signaling a potentially attractive therapeutic intervention point in different malignancies.
The Trk family of RTKs is frequently expressed in lung, breast, pancreatic and prostate cancers as well as in certain type of acute myelogenous leukemia and congenital fibrosarcoma. The tyrosine kinase activity of Trk is believed to promote the unregulated activation of cell proliferation machinery. It is believed that inhibitors of either TrkA, TrkB or TrkC kinases, individually or in combination, have utility against some of the most common cancers such as brain, melanoma, multiple myeloma, squamous cell, bladder, gastric, pancreatic, breast, head, neck, esophageal, prostate, colorectal, lung, renal, ovarian, gynecological, thyroid cancer, and certain type of hematological malignancies.
The Janus kinases (Jaks) modulate proliferation, survival and differentiation of a variety of cell types through integrating the signal transduction mediated by cytokine receptors. The Jak family of tyrosine kinases is comprised of the four family members, Tyk2, Jak1, Jak2 and Jak3. This kinase family shares several structural features including several Jak (JH) homology domains. The carboxy terminal JH1 domain contains the active kinase domain adjacent to a pseudokinase JH2 domain. Amino terminal to these domains are JH3-4 and JH5-7, which encode a domain similar to a SH2 and FERM domains, respectively. The SH2-like domain is not well characterized functionally amongst the Jak family members whereas the FERM domain, comprised of JH5-7, has been shown to mediate binding to cytokine receptors. Cytokine receptors are devoid of an intrinsic kinase activity and upon ligand binding, Jak family members are recruited to these receptors and are activated to phosphorylate tyrosine residues on the receptor complex and to downstream signaling molecules. A key downstream mediator of cytokine receptor signaling is the signal transduction and activator of transcription (STAT) proteins. There are seven mammalian STAT proteins (STAT1, 2, 3, 4, 5a, 5b, 6 and 7) which integrate signaling downstream of cytokine receptor activation. Upon recruitment to the cytokine receptor-Jak complex, STATs are tyrosine phosphorylated at the carboxy terminus by the Jak kinases. This phosphorylation results in the formation of STAT homo- or heterodimers through phosphorylated tyrosine and SH2 domain interactions. After activation through dimerization, the STAT proteins translocate the nucleus where they bind a response element in promoters to activate transcription of key genes involved in proliferation and differentiation. In addition to STAT regulation, Jak activation has also been reported to regulate other key growth/survival pathways including those mediated by IRS-1, Ras-MAPK, PI3K and Src kinase.
Alterations in Jak signal transduction has been reported in a variety of diseases. In leukemias, chromosomal rearrangements (Tel-Jak2) producing a constitutively active Jak kinase have been observed in atypical chronic myeloid leukemia. Activating point mutations have been observed in Jak3 in acute megakaryoblastic leukemia and Jak2 in acute myelogenous leukemia and in myeloproliferative disorders such as polcythaemia vera, essential thrombocythemia and myeloid metaplasia. A role for Jak kinases in solid tumors is also supported by the large number of reports of constitutive Stat3 activation in a wide variety of cancers. Elevated or constitutive STAT3 activity has been observed in breast, prostate, head/neck and melanoma tumor specimens and cell lines with pharmacological or genetic evidence for involvement of Jak activity in the observed STAT3 activation. In addition, Jak signaling has been implicated in malignant transformation through the activation of other key signaling pathways such as PI3K, Src, Bcl2 and Ras-MAPK. The communication of Jak signaling to these key pathways has the potential to broaden the involvement of this kinase family to a wide spectrum of malignancies.
Casein Kinase 2 (CK2) is a serine/threonine kinase that has been implicated in the regulation of stability of a number of oncogenic or tumor suppressor proteins. Proteins such as beta-catenin and c-Myc, that have been established as important components of signaling pathways in cancer cells, have been shown to be phosphorylated by CK2 and these phosphorylation events have been proposed to be required for the stabilization of these proteins. Tumor suppressor proteins, such as PML and PTEN, have also been shown to be phosphorylated by CK2, and these modifications have been suggested to promote degradation of these proteins. Inhibition of CK2 enzymatic activity, such as by small molecules, may lead to the destabilization of oncogene products as well as the stabilization of tumor suppressor proteins, resulting in an anti-proliferative effect on tumor cells.